To understand the importance of a good constructive design of the load cell housing, a short description of the measurement of tensile force and/or tensile stress will first be given. The deflector roll is journalled in a conventional manner in a bearing housing. Between the bearing housing and the base, that is, some kind of foundation, the load cell housing is placed. This substantially consists of two relatively stiff parallelepipedic yokes, one of which makes contact with the bearing housing and the other makes contact with the base. The yokes are connected together by a number of membranes and a transducer in such a way as to make possible a certain movability in the longitudinal direction of the yokes. The transducers are oriented such that only the force F.sub.r in the direction of movement, that is, normally in a horizontal direction, is measured. With knowledge of the angle of entry and the angle of departure of the continuous web at the deflector roll in relation to the direction of movement mentioned, the tensile force F.sub.d in the continuous web can be calculated in a simple manner. With knowledge of the cross section area of the continuous web in question, also the tensile stress can thus be determined.
The insertion of a transducer parallel to the above-mentioned membranes between the two yokes can normally be performed in such a way that no measurable forces arise in a direction parallel to the two yokes. Nor are there any major problems from the manufacturing point of view in making the outer plane sides of the two yokes plane-parallel.
An additional condition for obtaining a good accuracy is that the side of the bearing housing which faces the yoke of the load cell housing has a plane surface. The same demands are also placed on that surface of the base which faces the other yoke of the load cell housing. To be able to meet the given accuracy data, specified planeness requirements are often indicated.
It is also very important always to mount the load cell at an angle of exactly 90.degree. with the deflector roll to prevent any lateral forces from getting into the direction of measurement.
The attachment of the load cell housing to the bearing housing and to the base, respectively, is often carried out using screws which from the side of the bearing housing and the base, respectively, are screwed into threaded holes in the yokes. Although normally specified tightening moments are prescribed for the screwing, it has proved that the attachment of the load cell housing often influences the accuracy of measurement in a very negative way because internal bending moments and forces of no inconsiderable magnitude may then arise in the load cell housing. This manifests itself in such a way that a load cell senses a tensile force in a direction parallel to the yokes without there being any continuous web on the deflector roll.
Various designs of the yokes have been presented in order to reduce the above-mentioned problem. Instead of allowing the outer plane-parallel surfaces of the yokes to make close contact with the bearing housing and the base, respectively, it is possible to provide the plane surfaces of the yokes with narrow grooves parallel to the shaft of the deflector roll and placed on respective sides of the threaded holes. Such grooves are shown, inter alia, in figures in the ABB pamphlet PillowBlock tensiometer, A07-7505 E. Otherwise, this pamphlet shows the construction of a complete load cell and how this is integrated into a production plant for a continuous web where the tensile force or the tensile stress in the web need to be measured. The transducer used in this case consists of a magnetoelastic transducer which, by a suitable location of holes for an excitation and measuring winding, only measures the force in the longitudinal direction of the yokes.
Although the introduction of the above-mentioned grooves has entailed improvements in relation to the plane-parallel outer yoke sides, the problems have not disappeared entirely. The basic reason for these problems arising is, as mentioned above, that the tightening of the screws gives rise to internal bending moments and stresses in the yokes which may result in the load cell sensing a tensile force in a direction parallel to the yokes without there being any continuous web on the deflector roll. The technical explanation of the occurrence of bending moments in the yokes is that the contact surface around each screw in the present designs does not provide a symmetrical counter-support. By symmetrical counter-support is meant here that if the contact surface around each screw hole is conceived to consist of a large number of surface elements, then the sum of the "surface moments" of the surface elements, that is, the respective surface element area multiplied by its surface pressure and the distance to the screw centre, should be zero. Since neither the design solution with plane-parallel outer yoke sides, nor outer yoke sides with grooves exhibits a symmetrical counter-support, there will always be internal bending moments and associated mechanical stresses in the yokes.